A 2/1 set-up "scavenges" better at certain frequencys of pulses...That is, one escaping exhaust pulse helps "pull" the next one if they are resonating properly together....This requires 'tuned' lengths of header before the join, and works best when the pulse frequency's half-wave harmonics "overlap" to create a continuous/ steady pressure flow rate......IN THEORY......
MANY factors affect engine performance at certain rev speed ranges....This is just one of them.....